[cffi-devel] Re: Non-top level defcallback

[James Bielman]

Stephen Compall wrote:
> On Sun, 2007-05-20 at 19:39 -0700, Tomas Melchior wrote:
>> What platforms/implementations support this? The manual only says "some
>> Lisps". Also, is there anything analogous to lambda for defcallback? 
> 
> To refer to the text:
> 
> *Portability note:* `defcallback' will not work correctly on some Lisps
> if it's not a top-level form.
> 
> The point of this note is to say that CFFI doesn't guarantee that it
> will work, much as doing anything not defined by ANSI Common Lisp isn't
> guaranteed to work in portable Common Lisp code.  In fact, "some" may
> mean "none at all" now or at any future time.  In other words, don't do
> it.

I wonder if we even want to word this more strongly and outright declare
the results undefined or implementation-defined if at non top-level.

DEFCALLBACK is one place that we compromise and pass through to the
Lisp's underlying DEFCALLBACK macro, instead of using a functional
interface.  Unfortunately, this can allow implementation details to
leak through, as it does in this case.

I would love to see all supported Lisps providing the underlying
machinery to make a CALLBACK-LAMBDA happen, but I think the number
is currently quite low. :(  I did write a patch awhile back adding
this for (IIRC) SBCL and CLISP.  It should be in the cffi-devel list
archives.

With a widely supported CALLBACK-LAMBDA, we could implement DEFCALLBACK
in terms of CALLBACK-LAMBDA, as we do with DEFCFUN and FOREIGN-FUNCALL.
I'd like to hope we can get there someday...

James